Most of the previous studies of cardiac muscle mechanics have involved the use of the quick release technique to measure the elasticity and viscosity of papillary muscle. In order to continuously measure these mechanical properties during a complete contraction cycle, a sinusoidal forcing technique has been developed for the study of papillary muscle. In addition, a similar sinusoidal forcing technique has been developed which provides the first direct measurements of elasticity and viscosity in intact, isovolumically-contracting left ventricles of dogs. These techniques impose a sinusoidal volume or length change on the papillary muscle or ventricle and analyze the sinusoidal component of pressure or tension. These two sinusoidal-forcing techniques will be used to investigate: 1) The influence of changes in frequency of the forcing function on papillary muscle for the purpose of determining a multicomponent mechanical model consisting of linear or nonlinear elastic, viscous and inertial elements; 2) the influence of interventions known to change cardiac stiffness to determine whether the elastic or viscous component is more responsible for the stiffness change, and 3) the influence of interventions which can be imposed both on the papillary muscle and ventricle to compare elastic and viscous measurements made in the papillary muscle and intact left ventricle.